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Threaded fasteners are the most used type of fastener. Many threaded fasteners are assembled with nuts and washers.

Thread Standards: There are many different types of threads used for fasteners in industry, designed to be used for different jobs. Threads used on fasteners are manufactured to industry-established standards for uniformity. The most common thread standard is the unified standard, sometimes known as the American standard. Unified standards are used to establish thread series and thread classes.

Thread Series: Unified standards are established for three series of threads, depending on the number of threads per inch for a certain diameter of fastener.

1. Unified National Coarse (UNC) thread: Used for bolts, screws, nuts, and other general purposes. Fasteners with UNC threads are commonly used for rapid assembly or disassembly of parts and where corrosion or slight damage may occur.
2. Unified National Fine (UNF) thread: Used for bolts, screws, nuts, and other applications where a finer thread for a tighter fit is desired.
3. Unified National Extra Fine (UNEF) thread: Used on thin walled-tubes, nuts, ferrules, and couplings.

Thread Classes: The unified standards also establish thread classes. Classes 1A, 2A, and 3A apply to external threads only. Classes 1B, 2B, and 3B apply to internal threads only. Thread classes are distinguished from each other by the degrees of tolerance permitted. Classes 3A and 3B allow a minimum clearance and classes 1A and 1B allow a maximum clearance. Classes 2A and 2B are the most commonly used. Classes 3A and 3B are used when close tolerances are needed. Classes 1A and 1B are used where quick and easy assembly is needed and a large tolerance is acceptable.

Thread Identification is done using a standard method.

• Nominal Size: The nominal size is the approximate outer (or major) diameter of the fastener.
• Number of threads per inch (TPI): The TPI is standard for all diameters.
• Thread series symbol: This symbol indicates the Unified standard thread type (UNC, UNF, or UNEF).
• Thread Class Symbol: This symbol indicates the closeness of fit between the bolt threads and nut threads.
• Left-Hand Thread Symbol: This symbol is indicated by the letters LH. Unless threads are specified with the LH symbol, the threads are right-hand threads.

Thread Design: There are several types of thread design. The thread design depends on the purpose of the fastener. Power transmission threads are special threads that are used to move machine parts for adjusting, setting, and transmitting power. Three common power transmission threads are the square thread, acme thread, and buttress thread.

• Buttress Thread: has one side of the thread cut square and the other side cut at a slant. It has great strength along the thread axis in one direction only. This thread form is used to screw thin, tubular parts together.
• Square Thread: is the strongest and most useful of all thread forms. It is also the most difficult to make because its parallel sides must be accurately machined. Square threads are commonly used in precision measuring instruments, aerospace equipment and robotics due to their strength and ability to handle high-stress environments.
• Acme Threads: The Acme thread has replaced the square thread in most cases because it is easier to machine. Acme threads are commonly used in industrial machinery, power transmission systems, automotive industry, clamps and valves.

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